1. Field of the Invention
The present invention relates to a radio communication system, in particular, relates to a multi-hop radio communication system having a plurality of radio links.
2. Description of the Related Art
In a radio packet communication system such as a radio LAN (local area network) system and/or a mobile communication system, a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) has been known as a distributed control access system. In that system, it is tested for a predetermined duration whether a channel to which a signal is to be transmitted is available or busy, before a data packet is transmitted, thus, the possibility of collision of packets is decreased. That system is described in ANSI/IEEE Std802.11 Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specification, and is taken as MAC protocol of IEEE802.11 standard.
However, when said CSMA/CA system which uses a distributed control access system is used in a multi-hop radio communication network, a competition occurs in each link, and because re-transmission is inevitable in case of a competition, a delay time between end terminals has much variance. Further, when a link is full, the transmission must wait until the link becomes available, and therefore, a delay time increases, thus, a throughput performance is degraded.
However, when said CSMA/CA system which uses a distributed control access system is used in a multi-hop radio communication network, a competition occurs in each link, and because re-transmission is inevitable in case of a competition, a delay time between end terminals has much variance. Further, when a link is full, the transmission must wait until the link becomes available, and therefore, a delay time increases, thus, a thoughtput performance is degraded.
On the other hand, a polling access system which is a centralized control access system, and/or a polling/CSMA system which uses both a distributed control access system and a centralized control access system on time division basis, in which a master station controls transmission of a slave station, is free from a packet collision, and the delay and/or the delay variance is decreased, thus, the throughput performance and the value Q0S (quality of service) are achieved.
However, in case of a multi-hop radio network, a single master station cannot control all the radio stations. Therefore, a prior polling system, and/or a prior polling/CSMA system cannot be used in a multi-hop radio network.